Tapered threads have long been recognized as superior in forming couplings for bar and tube joints, particularly where tensile capabilities are important. Such taper thread joints in bars such as reinforcing bars used in concrete construction have been widely employed, an example being the LENTON brand coupler and coupling systems sold by Erico Products Inc. of Solon, Ohio or Erico BV of Tilburg, Holland. Such bars may be of substantial diameter and in some applications quite long or even bent. To cut tapered threads on such bars requires an expensive and complex thread cutting machine. For this reason smaller more portable thread cutting machines such as shown in Kies et al U.S. Pat. No. 4,526,496 have been developed. While such machines have proven effective in being able to taper thread the end of reinforcing bar, such threads are nonetheless cut.
It has also long been recognized that roll formed threads are superior to cut threads for most ferrous materials. Advantages of thread rolling are accuracy, uniformity, improved surface finish, and most importantly better tensile, shear and fatigue properties. The cold working of the bar end during thread rolling actually strengthens the threaded bar end in the area of the threads so that it then becomes possible to produce a bar joint having tensile strength approaching or greater than that of the bar alone.
Thread rolling is conventionally accomplished in machines employing flat dies, or two or three cylindrical dies. The rolling of tapered threads presents a more complex problem. Flat dies can be used where the part being threaded is relatively small such as self tapping screws as seen for example in U.S. Pats. Nos. 3,217,530; 3,896,656; 1,946,735; 1,971,917; 2,165,009; 2,183,688; 2,232,337; 2,293,930; 2,335,418; 2,348,850; 2,483,186; 3,176,491; 4,255,969; 4,546,639 and 4,563,890.
For larger parts such as pipe, tube or rods special rolling dies may be employed as seen for example in U.S. Pats. No. 859,643; 2,666,348 and 2,932,222.
Roll threading with essentially flat dies is limited in its ability to accommodate uniform fastener taper exceeding 2.degree.-3.degree., for example, when uniform pitch and thread form are required. Die speed cannot be coordinated with fastener surface speeds along the taper length during rolling. This results in twist or slip distortions between the large and small end of the tape. Slip results in stagger between the die and fastener when the part is formed. Additionally helix angles and thread tolerances are compromised. All such problems negate efficient assembly and strength development if the male threads are to be assembled with female threads prepared by a threading process which generates uniform pitch and thread form. While the flat die process may efficiently make self tapping screws, for example, it is not acceptable for producing a high strength precision bar or pipe joints.
The problem of forming rolled tapered thread on bar ends such as large bar, irregular surface bar such as concrete reinforcing bar, or earthing or electrical ground rods presents even further problems. For example rolled threads can often be more easily formed if or as the bar rotates. However if the bar is long, large or even bent this creates a problem. If the bar has irregular surfaces such as concrete reinforcing bar it is difficult to grip or position the bar so that its true centerline is located with respect to any thread forming dies.